Stretched bodies conventionally investigated and commercialized are each composed of: a polyolefin resin such as polyethylene or polypropylene; a polyester resin such as polyethylene terephthalate; or a polyamide resin such as nylon 6 or a nylon 6/66 copolymer. The stretching of an oxymethylene polymer has a long history, and, for example, Clark et al. have investigated a super-stretched fiber (see Non-patent Document 1). The oxymethylene polymer generally has the following characteristics: the polymer has a high degree of crystallinity; and is excellent in rigidity, strength, chemical resistance, and creep resistance. In addition, the oxymethylene polymer has been used mainly as a material for injection molding in a wide variety of applications including mechanism elements for automobiles and electrical apparatuses because the polymer crystallizes at a high rate. Further, the oxymethylene polymer has an extremely high ultimate theoretical strength, and the strength suggests that the polymer is turned into a body having a high strength and a high elastic modulus by orientation crystallization based on stretching.
However, the oxymethylene polymer has the following drawbacks: the polymer has a high degree of crystallinity, its melting point peak observed by DSC is extremely sharp, and the melting point and crystallization softening temperature of the polymer are so close to each other that it is hard to stretch the polymer. In addition, the fact that the polymer crystallizes at a high rate also largely constrains the stretch processing of the polymer.
In addition, in recent years, attention has been paid to, for example, the chemical resistance and abrasion resistance of oxymethylene polymers and copolymers, and, as a result, the development of new applications of a fiber and a structure obtained from the fiber as stretched materials as well as conventional injection- and extrusion-molded articles has been advanced. Accordingly, not only an improvement in technique for turning any such polymer or copolymer into a fiber by spinning or stretch processing but also an improvement in technique for the secondary processing of the polymer or copolymer has been requested.
A method involving the use of a specific oxymethylene polymer for spinning or stretch processing has been known (see Patent Document 1). The document describes a fiber using an oxymethylene copolymer that crystallizes at a low rate, and a method of producing the fiber. However, the document merely describes, in any one of its examples, an antioxidant or a heat stabilizer as an additive to be added to the oxymethylene copolymer, and refers to neither the kind nor amount of the additive nor an influence of the additive.    Non-Patent Document 1: polymer Engineering and Science, October, 1974, Vol. 14, No. 10, p. 682-686    Patent Document 1: JP 2003-089925 A